Such devices have operating frequencies of the order of a few hundred MHz to a few GHz, and are used in radiofrequency transmission circuits (mobile phone, radio link, wireless exchange of data, etc.) signal processing or in sensor systems.
More specifically, the field of the present invention relates to electrical isolation, the input and output of a component with a view mainly to achieving an impedance and/or mode conversion in acoustic filters, therefore filters using acoustic resonators, mainly bulk wave (Bulk Acoustic Wave or BAW) resonators. These filters are located in the radiofrequency stage of audio transmission systems, notably in mobile phone systems. FIG. 1 shows a block digram of an RF stage. The signal coming from the antenna Is is directed toward two channels, filtered in a first so-called reception channel by a bandpass filter 1, then amplified by a low-noise amplifier 2 LNA, for Low Noise Amplifier), and blocked, in parallel, in a second so-called emission channel by a filter 3 so that the signal does not interfere with the signal emitted in the direction of the antenna. The signal coming from the antenna is, by construction, referenced to ground, and therefore asymmetric. On the other hand, in order to limit the noise added to the signal by the amplification stage, LNAs usually have differential accesses where the signal is no longer referenced to ground, but rather propagates in two 180° out-of-phase versions along the transmission lines.
To ensure the connection between the part of the chain referenced to ground and the differential part, it is possible to use an element external to the filter, inserted between the filter and amplifier or between the antenna and filter, called a balun (for Balanced/Unbalanced). Baluns are systems which take up a lot of space and introduce losses. Baluns can indeed be built using transmission line sections which should have centimeter dimensions at the currently used frequencies for mobile telephony (on the order of a few GHz), or more generally using magnetic windings similar to transformers, therefore requiring large areas (a few mm2), and having non-negligible resistive losses.
CRF (Coupled Resonator Filter) filters, acoustically coupled filters, have already been proposed, enabling the mode conversion to be made G. G. Fattinger, J. Kaitila, R. Aigner and W. Nessler, Single-to-balanced Filters for Mobile Phones using Coupled Resonator BAW Technology, 2004 IEEE Ultrasonics Symposium, pp. 416-419, because by construction the input and output of the filter are electrically insulated: R. Thalhammer, M. M. Handtmann, J. Kaitila, W. Nessler, L. Elbrecht, Apparatus with acoustically coupled BAW resonators and a method for matching impedances, US patent 2009/0096549 A1, April 2009
FIGS. 2a and 2b show a filter with both mode-conversion and impedance-conversion in CRF technology, FIG. 2b showing the electrical connections of the resonators. The impedance conversion is obtained by the connection of two input resonators Re1 and Re2 and two output resonators, back to back, so as to increase by a factor of 4 the input impedance of the filter. The technological production of this filter is, however, very onerous, because it requires two resonators to be stacked on top of each other, and therefore a technology requiring the deposition of about fifteen layers and about twelve mask levels: C. Billard, N. Buffet, A. Reinhardt, G. Parat, S. Joblot and P. Bar, 200 mm Manufacturing Solution for Coupled Resonator Filters, Proceedings of the 39th European Solid-State Device Research Conference (ESSDERC 2009), pp. 133-136.
Likewise, it is possible to stack resonators R1 and R2 of different thicknesses M. L. Franck, R. C. Ruby, T. Jamneala, Bulk Acoustic Resonator Electrical Impedance Transformers, US patent 2009/0273415 A1, November 2009, as indicated in FIG. 3. These resonators are acoustically coupled via the layer(s) Cc located between them. The resonators are stacked on top of a membrane produced on top of a cavity Ca made at the surface of a substrate S. Impedance conversion is achieved through the difference in thicknesses, the lower and upper resonators having, for a given area, different capacitances, and therefore different impedances. This solution raises the same manufacturing problems as the CRFs, namely requiring numerous layers to be stacked and numerous mask levels to be used. In addition, the thicknesses are fixed within certain limits by acoustic constraints (the desired frequencies, the bandwidth, the coupling coefficients of the modes used, etc.), not allowing all the desired conversion ratios to be obtained.
Moreover, in order to provide a filter with sufficient selectivity, it is often necessary to use two filter sections that, for reasons of technological complexity, are often connected at the level of the lower resonators as shown in FIG. 4, and using two types of stacks comprising piezoelectric materials Epiezo1 and Epiezo2, isolated by Bragg mirror structures MR1 and MR2, all the stacks being produced on the surface of a substrate S. The areas of these sections are then defined by the respective input and output impedances, while the thicknesses of the lower and upper resonators are defined by acoustic considerations, as specified earlier. Thus, the two lower resonators do not have the same impedance, the latter being defined by the formula
                                          Z            res                    =                                    e              piezo                                      2              ⁢                              πɛ                piezo                            ⁢                              S                res                            ⁢                              f                0                                                    ,                            (        1        )            
where epiezo is the thickness of the piezoelectric layer, ∈piezo the dielectric constant of the piezoelectric material used, f0 the central frequency of the filter, all these quantities being common to the lower resonators, and Sres the corresponding area of each resonator. For this reason, electrical reflections are created inside the filter, which reflections degrade the transmission of the filter, and thus its performance.